Many power semiconductor circuits feature controllable power semiconductor switches connected in parallel to achieve a higher current switching capacity. In other applications, for instance in inverters, motor controllers, etc. the load paths from two controllable power semiconductor switches are connected in series so as to achieve one or more branches (“half bridges”) of the circuit, each branch including a high-side switch and a low-side switch. By using such branches, for example, power inverters including a three-phase bridge circuit (“6 pack) or a two-phase bridge circuit (“4 pack”) may be created.
Circuits may be designed symmetrically or also asymmetrically, e.g., in which more than one controllable power semiconductor switch are arranged on the low side as is the case e.g., with converters for switched reluctance machines (SRM) or with interleaved two-transistor forward (ITTF) converters.
For controlling each controllable power semiconductor switch a driver is provided which controls each power semiconductor switch involved in turning it ON or OFF. As a whole, the drivers together form a control circuit. Whilst when operating controllable power semiconductor switches high voltages exceeding e.g., 30 V may be dropped over their load paths, controlling controllable power semiconductor switches is done with relatively low voltages. Thus, in the case of a circuit branch in which high-side and low-side switches are connected in series separate power supplies are needed for controlling the high-side switches and low-side switches. Where a plurality of assemblies involving several circuit branches is provided, the emitter and/or the source contacts may be interconnected with a low impedance the same as in a parallel circuit of two or more controllable power semiconductor switches. In these cases a common low voltage source may be employed to supply the drivers of the low-side switches and the drivers of the controllable power semiconductor switches connected in parallel, respectively, since controlling the controllable power semiconductor switches can be done with a low voltage relative to the emitter or source contact of the respective switch. In such an arrangement the common low voltage source is connected to a central point of the power circuit.
This circuit has two serious drawbacks, however.
For one thing, because of voltages induced in parasitic inductances of the low-impedance conductors connecting the emitter or source contacts of the interconnected low-side switches of two or more circuit branches, ON/OFF switching of at least one controllable power semiconductor switch may result in a parasitic ON of one or more of the controllable power semiconductor switches unintentionally when their reference potential is pulled below the reference potential of the corresponding driver.
For another, the negative feedback at least for one of the low-side power switches is increased due to the inductance common to the load circuit (i.e., the power switches) and to the control circuit (i.e., the drivers) leading to increased power-up losses.
Another possibility is to connect the common power supply of the drivers for controlling the low-side switches at several points to the load circuit, in which case parts of the load current flow via redundant connections in the control circuit but which likewise adds to the power-up losses. Problems like this also materialize when operating several controllable power semiconductor switches connected in parallel.
Although, where pairs of branches, each controlled independently of the other, are involved, especially when the switching power required is small, the increase in the power-up losses is acceptable, additional costs materialize since the controllable power semiconductor switches and/or their heat sinks necessitate a larger rating.
And, although, especially where the switching power is higher, several galvanically decoupled power supplies may be put to use, this too adds to the complexity.
In parallel branched circuit assemblies resistances may be inserted in the connection between the driver and an auxiliary contact of the emitter or source contact—where the gate resistances are correspondingly reduced—but to adequately restrict the currents flowing via the auxiliary contact a prohibitively high resistance would be needed. Due to the drop in voltage across such a resistance any lack of symmetry in the switching action of the parallel controllable power semiconductor switches would be amplified and thus such a resistance would have to be restricted to roughly 10% of the gate resistance.
For these and other reasons there is a need for the present invention.
In the FIGs. like reference numerals identify like elements having the same or corresponding function. Where as illustrated in the FIGs. a permanent coupling of two components is made, for example, by using tracks, resistances, coils, capacitors, transformers, impedance components or the like any such permanent coupling may also be achieved as a switchable coupling, for example, by inserting a controllable power semiconductor switch. The two components can thus be coupled to each other even though this is not expressly mentioned.